1. Field of the Invention
The present invention relates to a compact zoom lens system and more particularly to a compact zoom lens system applicable to small-sized cameras such as video cameras and electronic still cameras.
2. Description of the Prior Art
In recent years, since packaging of electronic components has been improved increasingly and the rate of integration thereof has also been improved, the volume and weight of a lens system occupying in the main unit of a video camera or the like has become relatively large. Also, as to the cost, the lens system has become a bottleneck of overall reduction in the cost. In the case of the present-state video cameras and the like, smaller size, lighter weight and lower cost are absolute requirements, and how to miniaturize the lens system and how to configurate it more economically are important to realize the above-mentioned. There are many examples wherein as a result of pursuing the above, use of a zoom lens is given up and a fixed focal length lens system is adopted. However, this is sure to be able to realize the smaller size, lighter weight and lower cost, but the charm as a commodity is greatly reduced because of a fixed focal length. Needless to say, a focal length of a lens system can be altered by using a converter or an attachment, but such a thing has to be carried separately besides the camera, and further considering the whole including the camera and such an attachment, it is hard to be said that the smaller size, lighter weight and lower cost have been realized. Also, in recent years a system wherein a converter is incorporated in the camera main unit and the focal length is changed-over by a simple operation has been sometimes adopted for compact cameras and the like. However, considering the whole system, this also can never be said to be smaller in size, lighter in weight and lower in cost, and only a ratio of variation of the focal length of about two or less can be realized, and in photographing a moving picture, continuous change-over cannot be made during photographing, and therefore this also lacks the charm as a commodity.
Then, after all, the zoom lens system is considered to be applied, but the conventional ones offer aimed at a high zooming ratio, and therefore they were large in size and very high in cost. Among them, for a zoom lens system wherein the zooming ratio is reduced to about three and thereby compactness and cost reduction are pursued, the ones as disclosed in the Japanese Patent Laid-Open No. 261712/1986 and the Japanese Patent Laid-Open No. 21113/1987 can be enumerated, but the former is configurated with 11 lens elements, and thus the number of lenses is reduced, but the F number is as large as 2.8, while the latter has a F number of as small as 1.3, but has a configuration of 12 lens elements. Thus both are still large and heavy, and are hard to be said that the compactness and cost reduction have been satisfactorily attained. Furthermore, for an example wherein the zooming ratio is reduced to about two and thereby the cost reduction and compactness are pursued extremely, the one as disclosed in the Japanese Patent Laid-Open No. 143311/1983 is cited, but this system cannot be put in practical use in performance because of insufficient compensation of chromatic aberration. Also, from the viewpoint of chromatic aberration, the zooming ratio cannot be taken larger than that value. On the other hand, a system employing a lens for silver nitrate film such as a single-lens reflex lens is shown in the Japanese Patent Publication No. 27849/1970, but this is very large in F number, and therefore cannot be applied to the fields such as video cameras where a bright lens is required.
Particularly, in the field such as video cameras, there are many opportunities of photographing in the very dark state without using an auxiliary light such as a flash, and the minimum F number of lens system is a very important factor along with the sensitivity of the photographing device.
The smaller minimum F number and the compactness and low cost are requirements contrary to each other, and at the present state, a compact zoom lens system satisfactorily meeting the both requirements is not yet provided.